Many of various mobile electronics equipments such as cell phones, mobile information terminals PDA, note PCs, mobile audio/video players, digital cameras, video cameras, etc. have DC-DC converters as devices for converting power source voltage to operation voltage. As one example of the DC-DC converter circuits, FIG. 26 shows a step-down DC-DC converter circuit comprising an input capacitor Cin, an output capacitor Cout, an output inductor Lout, and a semiconductor integrated circuit IC comprising a control circuit CC, etc. In the step-down DC-DC converter, switching devices (for instance, field effect transistors) in the semiconductor integrated circuit IC are switched according to a control signal, to lower a DC input voltage Vin to an output voltage Vout [=Ton/(Ton+Toff)×Vin], wherein Ton is a time period in which the switching devices are turned on, and Toff is a time period in which the switching devices are turned off. The control of a Ton/Toff ratio makes it possible to stably provide a constant output voltage Vout even from the varying input voltage Vin.
FIG. 27 shows one example of switching circuits in the semiconductor integrated circuit IC. A control circuit CC for controlling the switching operation of alternately turning on and off MOS transistors SW1, SW2 is formed on a silicon semiconductor substrate. Because the control circuit CC per se is known, its explanation will be omitted. The input capacitor Cin stabilizes the input voltage Vin in a transient state while preventing spike voltage, though it may be omitted. A filter circuit (smoothing circuit) for outputting the DC voltage Vout is a combination of an output inductor Lout for storing and discharging current energy, and an output capacitor Cout for storing and discharging voltage energy.
To reduce the size of DC-DC converters, increasing higher switching frequencies have been getting used, and DC-DC converters switched at a frequency of 1 MHz are now used. Also, higher speed and function as well as lower operation voltage and higher current have been getting used for semiconductor devices such as CPUs, requiring that DC-DC converters provide lower-voltage, higher-current output. However, lower operation voltage makes the semiconductor devices susceptible to the output voltage fluctuation (ripple) of the DC-DC converters. Proposed to prevent this are DC-DC converters having switching frequencies further increased to about 2-10 MHz.
FIG. 28 shows one example of step-up DC-DC converter circuits. This DC-DC converter comprises an input inductor Lin, an output capacitor Cout, and a semiconductor integrated circuit IC comprising a control circuit CC. By controlling time periods of turning on and off switching devices, a high output voltage Vout is obtained from an input voltage Vin.
As another example of DC-DC converters, FIG. 29 shows a multi-phase, step-down DC-DC converter comprising an input capacitor Cin, an output capacitor Cout, output inductors Lout1, Lout2, and a semiconductor integrated circuit IC comprising a control circuit CC. The multi-phase DC-DC converter comprises pluralities of switching circuits, which are operated with different phases lest that their switching periods overlap, current outputs from the switching circuits being combined by a smoothing circuit. This flows low current in each path and suppresses the ripple.
Because such a circuit has as large an apparent operation frequency as n times the switching frequency, the switching frequency can be made 1/n. Accordingly, output inductors Lout1, Lout2 having excellent high-frequency characteristics need not be used, but high-Q-value inductors may be used, expanding the freedom of selecting devices. Multi-phase DC-DC converters are operated with a phase difference of 180° in the case of a two-phase type, and with a phase difference of 120° in the case of a three-phase type. Increase in the number m of phases increases the number of inductors, while reducing inductance necessary for each inductor to 1/m. As a result, small inductors and high-Q-value inductors can be used, thereby avoiding the DC-DC converters from becoming extremely large.
Such a DC-DC converter is generally constituted by a semiconductor integrated circuit IC (active element) comprising switching devices and a control circuit CC, and passive elements such as an inductor and a capacitor, etc., which are mounted on a circuit board such as a printed circuit board having connecting lines, etc. as a discrete circuit. Among the passive elements, an inductor that should have inductance of at least several μH is large, occupying a large area of the circuit board, and it is not easy to reduce its size. Further, because the circuit board should have a line pattern connecting the active element and the passive elements, there is a limit to miniaturize the DC-DC converter constituted as a discrete circuit.
For size reduction, the integration of a semiconductor integrated circuit and inductors was proposed. For instance, JP 2004-063676 A discloses a DC-DC converter comprising a printed circuit board PB having connecting terminals (stud terminals) ST, a chip inductor CI connected to the terminals ST, and a semiconductor integrated circuit IC mounted on the printed circuit board PB, the chip inductor CI and the semiconductor integrated circuit IC being vertically overlapping (see FIG. 30). JP 2005-124271 A discloses a DC-DC converter comprising a semiconductor integrated circuit IC and a smoothing capacitor SC disposed on an upper surface of a glass-epoxy, multi-layer substrate MS containing a smoothing inductor SI, the smoothing inductor SI, the smoothing capacitor SC and the semiconductor integrated circuit IC being connected by wiring on the multi-layer substrate MS (see FIG. 31).
Having no wiring patterns for connecting the active elements and passive elements, the DC-DC converters of JP 2004-063676 A and JP 2005-124271 A need only small mounting areas, but they suffer the following problems.
The first problem is that a chip inductor CI that should have inductance of several μH cannot be miniaturized like a semiconductor integrated circuit IC. Because the DC-DC converter of JP 2004-063676 A should have a slightly larger printed circuit board PB than a large chip inductor CI, it cannot be made small, but is thick due to the printed circuit board PB and the stud terminals ST. In the DC-DC converter of JP 2005-124271 A, the inductor SI generating a magnetic flux in a transverse direction of the glass-epoxy, multi-layer substrate MS should have a large number of laminated coil turns to have a desired inductance because of a small magnetic path cross section, resulting in difficulty in its miniaturization. Also, increase in the number of laminated coil turns results in larger DC resistance, which decreases an output voltage Vout. Thus, the DC-DC converter has low conversion efficiency.
The second problem is a magnetic flux leaking from an inductor. Because the semiconductor integrated circuit is disposed close to the inductor in the DC-DC converters of JP 2004-063676 A and JP 2005-124271 A, a magnetic flux leaking from the inductor should be reduced sufficiently. FIG. 32 shows a magnetic flux generated from a laminated inductor comprising electric insulating layers (dummy insulating layers) and coil patterns alternately laminated, ends of the coil patterns being successively connected to form the laminated coil, and the outermost end being connected to an external electrode. The magnetic flux generated from the laminated coil passes through the dummy insulating layers, and partially leaks when the dummy insulating layers are non-magnetic or not sufficiently thick. The leaked magnetic flux acts as noise to nearby electronic parts such as a semiconductor integrated circuit, etc. When the multi-layer substrate has line patterns connecting an active element to passive elements as in JP 2005-124271 A, the leaked magnetic flux induces current in the connecting patterns, thereby generating noise.
To prevent the magnetic flux leakage, the dummy insulating layer is made thicker. Also, to prevent a magnetic flux from leaking to side surfaces, the laminated coil should be provided with a smaller diameter, the dummy insulating layer should be thicker, or the laminated coil should have a larger surrounding region. However, when the laminated coil has a smaller diameter, the number of layers for coil patterns should be larger accordingly, resulting in a thicker laminated inductor, a larger number of steps, and larger DC resistance. A thicker dummy insulating layer leads to a thicker laminated inductor. Also, a larger region around the laminated coil makes a multi-layer substrate larger.
The third problem is the parasitic inductance. Connecting lines of circuit elements per se have parasitic inductance. In the step-down DC-DC converter shown in FIG. 27, for instance, parasitic inductance series-connected to a source of a transistor switch SW1 generates a counter electromotive force in a connecting line having parasitic inductance when the transistor switch SW1 is OFF, increasing voltage at a source terminal of the transistor switch SW1. This results in a large turn-on loss, which leads to lower conversion efficiency. When a line pattern is formed on a printed circuit board as in JP 2004-063676 A and JP 2005-124271 A, as large inductance as to lower conversion efficiency is not generated. But when line pattern is formed on the multi-layer substrate 10 using a magnetic material, large parasitic inductance is likely generated.
The fourth problem is heat generated by a semiconductor integrated circuit. Insufficient heat dissipation is likely to cause thermal runaway in the transistor switch. Also, when a magnetic material is used for insulating layers constituting an inductor, the inductance varies, resulting in lower conversion efficiency.